High frequency energy interchange device



Jan. 5, 1965 E. J. NALOS 3,164,742

HIGH FREQUENCY ENERGY INTERCHANGE DEVICE Filed Dec. 27. 1960 s Sheets-Sheet 1 INVENTORZ ERVIN J. NALOS 4 HIS ATTORNEY.

, 3 Sheets-Sheet 2 E. J. NALOS HIGH FREQUENCY ENERGY INTERCHANGE DEVICE F|G.3. I2 g INVENTOR t ERVIN J. NALOS mzflm Jan. 5, 1965 Filed Dec. 27. 1960 O O O o 9 w 7 6 5 HIS ATTORNEY.

FIG].

VOLTAGE Kv.

Jan. 5, 1965 E. J. NALOS HIGH FREQUENCY ENERGY INTERCHANGE DEVI-CE Filed Dec. 27, 1960 3 Sheets-Sheet 3 I .INVENTORI ERVIN J. NALOS,

HIS ATTORNEY.

United States Patent F This invention relates generally to the class of devices which depend upon an interchange of energy between a stream of electrons and a radio frequency field to provide amplification and/or oscillations. Klystrons and traveling-wave tubes are examples of such devices. in particular, the present invention relates to an arrange ment for focusing the electron stream in such devices.

The phenomenon which is of primary interest in connection with high frequency energy interchange devices of the type under consideration is that the unidirectional energy in an electron stream may be converted to radio frequency energy in an electromagnetic wave. The mechanism for this energy conversion is dependent upon the fact that when an electron stream and a radio frequency electromagnetic field interact, the velocity of the electrons in the stream is alternately increased and reduced with resultant beam launching. In the energy interchange device referred to as a traveling-wave tube, amplification is generally provided by continuous int-eraction between the electron stream and a'radio frequency field. Amplification of the radio frequency field results from the fact that if the electron stream and radio fre quency field are of the proper character to interact, the electron stream will impart more energy to the radio frequency field than it extracts therefrom. In the device commonly referred to as a klystron, the amplification takes place in discrete regions. The electrons in the stream for the ldystron are bunched by interaction with a radio frequency field and the bunched electron stream is subsequently utilizedto generate an electromagnetic oscillation. in this manner, radio frequency fields are induced which are considerably stronger than the orig inal stream bunching fields, the energy being obtained at the expense of the unidirectional energy of the stream.

in order for an appreciable energy interchange to take piece between an electron stream and a radio frequency field, the electron stream must pass through a region containing the radio frequency field (commonly called the region of interaction) and some means must be provided for extracting amplified radio frequency power. The apparatus required for an amplifier utilizing the interaction between an electron stream and radio frequency field includes a circuit which will conduct radio frequenc current and some means to produce a stream of electrons in the region of the radio frequency field produced by the radio frequency current. The problems involved in producing and focusing or shaping a stream of electrons in an interaction region and producing a radio frequency field in the same region which is of a proper character to interact with the electron stream are common to the general class of devices to which this invention relates. The manner in which these problems are solved by the present inven tion is described and illustrated in connection with high power traveling-wave tubes since the present invention finds particular utility in connection with such a device.

For energy interchange to take place in a travelingwave tube in an appreciable amount, two conditions must be met: first, the electron stream must pass through a region containing the radio frequency field (commonly called the region of interaction) and, second, the velocity of the electron stream must be of the same order of magnitude as the velocity of a component of the radio 3,164,742 Patented Jan. 5, 1 965 frequency field in the direction of travel of the electron stream in the region of interaction.

Thus, a traveling-Wave tube includes an electron gun for producing a stream of electrons in an interaction region and a radio frequency circuit for producing the required radio frequency field in the region of interaction. The speed of electrons in an electron stream produced by such a gun depends upon the accelerating voltage applied to the gun. In general, the speed of electrons from such guns is much less than the speed of light. For example, the speed of electrons from a gun utilizing an accelerating voltage of 2500 volts is approximately one-tenth the speed of light and the speed of electrons from a gun utilizing an accelerating voltage of 80,000 volts is ap proximately one-half the speed of light. Since current tnavels along a conductor at approximately the speed of light and its associated electric and magnetic fields are propagated down the conductor at the same speed, some means must be provided either to increase the velocity of the electrons in the stream or to reduce the speed of propagation of the radio frequency field in the direction of flow of the electron stream if the second condition for energy interchange set forth above is to be met.

The first condition for energy interchange set forth above is that the electron stream produced by the electron gun must pass through the interaction region in close proximity to the radio frequency field. .For most effective operation, electrons from the stream should be confined to the interaction area throughout its length.

ince the space charge of the electrons in the stream tends to force them apart, some means must be provided to focus the stream throughout the interaction area to prevent the electrons from spreading and passing out of the interaction area. A common method of focusing is to provide a unidirectional magnetic field having lines of force in the direction of the electron stream by employing a solenoid which surrounds the entire traveling-wave tube structure and produces an axial magnetic field. Such a field tends to confine (focus) the electron stream, but where the electron beam current is relatively large, the main problem is one of producing a magnetic field along the stream which is of sufficient strength to perform the focusing function. In order to produce such a held with a solenoid, the solenoid must be extremely large and considerable power must be applied. A means of reducing the weight of the magnetic focusing structure and completely eliminating the associated power consumption is offered by the use of permanent magnets in a technique known as periodic magnetic focusing. I This approach is so named because the magnetic field Which is produced along the electron stream path to focus the stream varies in a periodic fashion (usually sinusoidally). As reported by I. T. Mendell, C. F. Quate, and W. H. Yokum in an article entitled, Electron Beam Focusing With Periodic Permanent Magnet Fields, which appears in the Troceeauigs of the ll cii, May 1954, pp. 800 through 810, inclusive, the Weight may be reduced by a factor of 30 when using a periodic magnetic field as compared with a continuous or uniformal axial field. eriodic focusing as contemplated by the authors of this article is known as the series ring approach since the axial magnetic field is produced by surrounding the traveling-wave tube structure with a series of annular magnets placed end in such a manner that their north poles are adjacent and their south poles are adjacent.

Thus, the magnetic field along the axis of the rings varies in a periodic manner.

Although the series ring approach to periodic magnetic focusing has definite advantages, where it can be used, its use is limited to focusing electron streams of relatively low density, i.e., low perveancer This is true since a high flux density is required to focus a high density electron stream and the flux density which can be produced on the axis of a series ring structure using available magnetic materials is not high enough. The maximum flux density which can be produced along the axis of a series ring type magnetic focusing structure is established by the coercive force of the magnetic material used. Even the best magnetic materials available at the present time, certainly those which it is commercially feasible to use, have such low coercive forces that the peak flux density which can be obtained on the axis of a series ring focusing structure is not sufficient to focus a high current density electron stream. Consequently, the series ring approach to periodic magnetic focusing of high perveance streams is not practical for many applications at the present time.

Even if magnetic materials of unlimited coercive force were available, the series ring approach has a number of other disadvantages which must be overcome. For example, the magnetic field along the axis of the focusing structure does not vary in a uniform manner throughout the length of the structure unless magnetic rings are available which have precisely the same characteristics. Such rings are extremely hard to produce.

There are many other approaches to focusing electron streams with a periodic magnetic field. The arrangement referred to as a cross web focusing structure is one of the most successful periodic permanent magnet focusing systems. A typical cross web focusing structure is described in detail in connection with FIGURE 1 subsequently. However, in order to provide an understanding of problems associated with this arrangement it is briefly described herein connection with that figure. The magnetic poles comprise two sets of elongated magnetic bars (12) having rectangular cross sections. The individual bars of each set are interposed between bars of the opposite set with their axes perpendicular to such bars. Thus, the bars'are so arranged that an end view of the arrangement has the appearance of an X with one leg of the X made up of bars of one set and the opposite leg of the X made up of the bars of the opposite set.

The desired magnetic field is then developed along the axis of this structure by applying magneto motive force between the ends of the two sets of bars in such a manner that a magnetic north pole is applied to opposite ends of one set and a magnetic south pole is applied to opposite ends of the opposite set. As illustrated in FIGURES l and 2 the magnetomotive force is supplied by horseshoe magnets which bridge the corresponding ends of opposite sets of bars.

With this arrangement all of the magnetic gaps in the structure are connected in parallel to a magnetic field generating means. Thus, the cross web structure substantially eliminates the coercive force of available magnetic material as a limiting factor on the flux density of the periodic permanent magnetic focusing structure and eliminates the necessity of matching the characteristics of a plurality of individual magnets.

Although the cross web focusing arrangement provides a high peak and average magnetic field along the axis of the structure asymmetries of the field are frequently encountered. The asymmetries may tend to make the electron stream asymmetrical in cross section. If the electron stream is not symmetrical in cross section, some of the electrons may be intercepted by the circuit. This causes undue heating of the circuit andreduces the power of the electron stream which would be available for transfer to the radio frequency electromagnetic fields.

Accordingly, it is an object of this invention to provide a structure for developing a periodic focusing field for electron streams wherein the flux density is not limited by the coercive force of available magnetic materials and magnetic asymmetries are essentially non-existent along the axis of the structure.

Briefly stated, in accordance with this invention, a focusing structure is employed which incorporates both se- 4 ries ring and cross web arrangements described above to produce a periodic magnetic field which is particularly suited to focusing high perveance electron streams.

As previously indicated, one component of travelingwave tubes generally is the slow-wave circuit. One such slow-wave circuit is a conductor wound in a helical configuration with the axis of the helix in a direction parallel to the direction of the electron stream. The helix is an excellent slow-wave circuit with many desirable characteristics; however, as the voltage and power handling capabilities of the tube are increased, it becomes necessary to open the structure more and more to obtain interaction. A limit is reached wherein the helical circuit is too open to be practical. Therefore, other slow-wave circuit structures have been utilized at high voltages. As an alternative to the helical circuit, more complex circuits such as cross wound helices or multifilar helices offer possibilities of avoiding certain difiiculties inherent in helices but these circuits still suffer from power handling limitations which make them unsuitable for high power application. As a consequence, circuits which consist of all metal envelopes with short metallic tabs from the interaction region to the exterior have been developed for traveling-wave tubes which deliver powers in the megawatt range. Such structures are called periodical loaded waveguides. Two such structures are analyzed in detail in an article by M. Chodorow and E. I. Nalos, entitled, The Design of High Power Traveling-Wave Tubes, which appears in the Proceedings of the IRE, volume 44, No. 5, May 1959, pp. 649 to 659. Although the magnetic current structure of the present invention is suitable for use with any type of radio frequency interaction circuit which fits the specified geometry of the focusing scheme, it finds particular application in connection with the periodically loaded waveguide structure. There are several reasons for the particular applicability of the present invention to this type of radio frequency circuit but the most important is that the radio frequency electric circuit structure and the magnetic circuit structure may be combined in such a manner that the magnetic focusing field may be brought into the structure on the electric circuit and concentrated along the axis of the tube.

It is therefore another object of this invention to provide a magnetic focusing structure for focusing high density electron streams which structure may be combined with the structure of a periodically loaded electric circuit.

The novel features which are believed to be charac teristic of this invention are set forth with particularity in the appended claims. The invention itself however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

FIGURE 1 is a partially broken away perspective view illustrating one embodiment of the invention wherein the radio frequency and magnetic circuits are combined;

FIGURE 2 is an end view of the structure illustrated in FEGURE 1;

FIGURE 3 is an enlarged sectional view taken along the longitudinal axis of FIGURE la and showing a portion of the interaction region as indicated by the lines 3-3 of FIGURE 2;

FEGURE 4 represents a plot of the flux density in gauss produced at the center of the structure of FIGURES l, 2, and 3 as measured along the longitudinal axis (abscissa);

FIGURE 5 is a partial central longitudinal section taken through a traveling-wave tube to illustrate the use of the particular magnetic circuit structure as a focusing means for a high power traveling-wave tube;

FIGURE 6 is a portion of a transverse section through the tube taken on line 6-6 of FIGURE 5; and,

FIGURE 7 shows plots of the percent beam transmission (ordinate) versus the beam accelerating voltage (abscissa) for the traveling-wave tube of ETGURE 5 (curve I) and for the same structure without the series magnetic rings (curve II).

Referring specifically to FIGURES l and 2 of the drawings, a cylindrical slow-wave structure is provided for propagating radio frequency waves therethrough at a velocity which is appropriate for interacting with an electron stream projected along the axis of the structure. In order to simplify the description and drawing of the radio frequency electric circuit and the magnetic circuit, the coupling means for introducing the radio frequency power and for removing the radio frequency power from the slow-wave structure is not illustrated in FIGURES l, 2, and 3. However, this aspect of the traveling-wave tube is illustrated and discussed in detail in connection with the description of FIGURE 5.

The slow-wave circuit 10 includes a cylindrical waveguide 11 which is loaded by means of a series of short metallic tabs 12 extending from outside the cylindrical structure of the waveguide 11 into the interaction region Thus, the particular slow-Wave circuit 10 illustrated is known as a periodically loaded slow-wave structure. As illustrated, the loading tabs 12 are metallic conducting bars which extend entirely through the waveguide and have centrally located apertures 13 therethrough. The apertures 13 are aligned along the axis of the waveguide structure to permit passage of an electron stream. Thus, each bar 12 extends through the cylindrical Waveguide structure and defines a central opening for passage of an electron stream and a pair of segments of a circle 14 between each side of the bar and the inner surface of the cylindrical waveguide structure. These open segments Jld provide a coupling between sections of the waveguide structure. In a conventional periodically loaded waveguide structure, the longitudinal axes of all metallic loading tabs 12 intersect the axis of the waveguide structure 11 at right angles and all lie in a single plane which plane includes the longitudinal axis of the waveguide structure 11. However, in order to provide the unitary magnetic and radio frequency circuit of the present invention, the loading tabs 12 are so arranged that they intersect the axis of the waveguide structure ill at an angle of 90 but, as illustrated, the longitudinal axis of each succeeding tab 12; along the waveguide structure is rotated 90 with respect to the preceding tab 12. Thus, when looking down the axis of the cylindrical waveguide structure }lll (end View of FIGURE 2) the loading tabs 12 define an X. The angle of 90 was selected only as a matter of convenience in fabricating the structure and it should be particularly noted that the angle of the alternate tabs 12 is not critical and it could be, for example, 60 or 80 rather than 90.

In efi'ect, the series of loading tabs 12 in the waveguide structure 10 define two sets of elongated bars 9 and 15. The loading tabs 12 in each set have their longitudinal axes in a given plane. For example, when looking at FIGURE 2, the longitudinal axes of the tabs 12 of one set 9 are in the plane which slopes from the upper left to the lower right cutting the longitudinal axis of the waveguide structure ll and the longitudinal axes of the tabs 12 of the opposite set 15 are in a plane sloping from the upper right to t lower left, also cutting the longitudinal axis of the waveguide structure ll. Since the periodically loaded circuit 10 illustrated is of the type known as a spatial harmonic structure, cylindrical tubelilce members 16 are provided within the apertures 13 in each of the metallic tabs 12 along the waveguide structure 10 in such a manner that their axes are concentric with the axis of the structure itself. The tubular members 16, known as drift tubes, are provided for the purpose of shaping the electric field in the interaction region in such a manner that interaction with the desired harmonic is most advantageous. The drift tubes it? define a series of centrally located coaxial tube spaced apart along the length of the structure lb. In practice the drift tubes 16 are formed by milling both sides of the portion of the tabs 12 which are inside the waveguide 11 in such a manner that the drift tubes are of the same material as the tabs 12.

in order to produce a periodic magnetic field along the length of the waveguide ill, the loading tabs 12 are brought out through the outer surface of the cylindrical waveguide 11 and coupled to a source of magnetom-otive force. As described in more detail subsequently the magnetomotive force is supplied by two sources. One source is a pair of horseshoe permanent magnets 17 and 18 coupled to the ends of the loading tabs l2 and a series of ring magnets 25 interposed between the loading tabs 12 outside of the waveguide llll. As illustrated in the embodiment of FIGURES l and 2, the loading tabs 12 which are brought out through the top of the cylindrical guide l0 are planed oil in such a manner that their ends define or lie in a common plane along the upper surface of the structure and the ends of the bars along the lower surface of the structure are planed oil so that they also lie in a common plane which is parallel to the plane defined by their upper ends. The ends of the bars are planed in this manner so that permanent magnets 1'7 and 1d utilized as the magnetic field generating means to produce periodic magnetic field along the length of the structure may be of the common horseshoe variety. In order to couple these permanent horseshoe magnets 17 and it; to the structure of the waveguide, a bar of magnetically permeable material is placed along each outer end of each of the crossed sets of loading tabs 9 and 15. Thus, there are four magnetically permeable bars 23, 21, 22, and 23 (FIGURE 2) which act as magnetic bus bars, each extending along one end of one set of the loading tabs 12 of the structure.

The magnetic field produced along the axis of the structure is made periodic (or reversing) by placing the flat ends of the horseshoe magnets 17 and 18 on the bus bars 20 and 21, and 22 and 23, respectively, in such a manner that the north pole N of each of the horseshoe magnets is coupled to bus bars 2t and 22 of a given set of loading tabs 9 and the south pole S horseshoe magnets is placed on the bus bars 21 and 23 Which extend along the opposite set of loading tabs 15. Since the loading tabs in one set S are interposed between the loading tabs of the opposite set 15, the magnetic field produced by the horseshoe magnets 17 and 18 is brought into the axis of the structure it) in such a manner as to produce a periodic magnetic field therealong. For example, since the magnetic lines of force produced by a magnet are considered to have the direction north to south and since alternate loading tabs of the magnetic circuit are connected to the north poles N of the permanent magnets 1'7 and 18 (i.e. the set of loading tabs 9), lines of force are produced which leave the set of loading tabs and are directed toward the loading tabs of the other set 15. This is most clearly seen in EiG-URE 3 wherein the lines of force along the structure are illustrated.

The exact configuration of the magnetic field along the axis of the structure is determined to a large extent by the spacing bet een the drift tubes 16. The spacing is not highly critical but it has been found that the best results are obtained when the flux distribution along the axis of the structure is substantially sinusoidal such as the distribution illustrated in FIGURE 4 of the drawings. In practice the flux distribution illustrated in FIGURE 4 was obtained with the drift tubes to extending over approximately 50% of the length of the structure (spacing between thedrit tubes is such that it takes up the remaining 50% The cross web focusing thus far described in detail is a structure which has been successfully employed in high power traveling-wave tubes utilizing electron streams of high density and high pe'rvcance. The structure is equally useful for any application where a periodic of each of the magnetic field is desired. A high power multicavity klystr-on, which also uses drift tubes, is one specific example and there are many others. The radio frequency circuit described does not have to be present and does not have to be utilized. For example, if it is desirable to produce a periodic magnetic field in an electron tube, the tube may be inserted in the apertures 13 of the magnetic loading tabs 12 and a periodic magnetic field is thus introduced along the axis of the tube. Obviously if this situation applies, there is no need to use the waveguide 11 as part of the structure.

There are certain advantages to utilizing the cross web structure in the traveling-wave tube as described. The first and most apparent advantage is that the radio frequency circuit and the magnetic circuit structure is unitary. Another advantage is that the magnetic field is brought into the interior of the structure it) and produced along the axis of the radio frequency circuit so that the magnetic circuit is of highest efiiciency (this is also true in the case of a klystron). Other advantages of the magnetic circuit structure apply generally whether or not the magnetic and radio frequency electric circuits are combined to produce the most beneficial results in this arrangement. For example, the magnetic circuit configuration is such that the magnetic flux is produced along the axis of the structure 10 by means of a single pair of magnets 17 and 18 and all of the gaps in the periodic magnetic circuit are operated in parallel, thus assuring a high degree of uniformity in the peak flux density of the magnetic field along the axis of the structure. Additionally, the magnetic circuits may be of any desired length since the horseshoe magnets 17 and 18 themselves can have any desired length without interfering with the circuit structure. Consequently, the coercive force of available magnetic materials does not limit the flux density which can be obtained along the axis of the magnetic circuit. The only limit placed on the peak flux density obtainable is the saturation of the magnetic circuit itself. The practical result of this improvement makes the difference between an arrangement which is of interest only for experimental purposes and one which is of practical value in high power tubes. The discussion given below illustrates this point.

The perveance S of an electron stream is determined by the current density of the stream and the stream accelerating voltage. Perveance is defined by the equation:

where I is the current in the electron stream (amperes) V is the stream accelerating voltage The magnetic field required to focus an electron stream is given by the following equation:

where B is the peak value of the periodic magnetic field in gauss and the minimum magnetic field required to focus the stream. k is a constant which depends upon the thermionic con-tent of the periodic magnetic field, and r is the radius of the electron stream.

From Equation 2 it can be seen that if the peak value of the periodic magnetic field is increased by a factor of (n) it will focus a stream having a perveance S which is greater by a factor of (n)? By way of example, about the highest peak flux density which has been obtained by a conventional permanent magnet periodic focusing structure is 1,000 gauss. It has been found that a peak flux density along the axis of the tube illustrated in FIGURE 5 can easily be made 3,000 gauss utilizing the focusing structure of the present invention. Thus, the new focusing structure makes it possible to focus a beam having a perveance greater by a factor of 3 or 9 at a given beam accelerating voltage and beam radius.

With all of its advantages, there are still limitations on the use of the cross web structure described. One of the primary problems is that of beam transmission. That is, the percentage of electrons in the stream which travel the full length of the structure may be quite low. For example, 70% transmission is not an unusual figure. This means that 30% of the electrons are collected on the circuit and thus serve no useful purpose. In addition the electrons which intercept the circuit generate heat which must be dissipated.

Analyses of the electron streams produced by cross web focusing structures indicate that the poor electron transmission is a result of a tendency of the magnetic field produced to distort the cross section of the electron stream. It has been found that the cross section of an electron stream in such a focusing structure tends to be eliptical.

The addition of the ring magnets 25 to the structure virtually eliminates asymmetries of the magnetic field on the axis of the structure and increases the peak magnetic flux obtainable. The ring magnets 25 each have the shape of a right circular cylinder and is magnetized between opposite faces so that one face is a magnetic north pole and the other is a magnetic south. The ring magnets are positioned around the waveguide 11 between loading tabs 12 concentrically with respect to the longitudinal axis of the structure in such a manner that the face of each of the ring magnets 25 which provides a north magnetic pole is adjacent one of the set of tabs 9 which is magnetically energized by a magnetic north pole of the horseshoe magnets 17 and 18 and the opposite faces contact the opposite set of tabs 15. In other words, ring magnets 25 are positioned with their polarities alternating in progression down the length of the structure.

As illustrated, the circular waveguide acts as a mandrel on which the ring magnets are fitted. This, of course, is not a requirement of the structure.

The efiectiveness of the ring magnets 25 in eliminating magnetic asymmetries may be seen from the curves of FIGURE 7. Curve I shows percent electron stream transmission for different stream accelerating voltages for the traveling-wave tube of FIGURES 5 and 6 without thering magnets 25 and curve II shows the percent transmission when the ring magnets are added. The increase in transmission for most beam voltages is better than 20% FIGURE 5 illustrates one embodiment of a complete traveling-wave tube 25 which utilizes the periodic magnetic focusing structure and slow-wave circuit illustrated in FIGURES l, 2, and 3 and described in connection with those figures. Consequently, portions of the structure illustrated in FIGURE 5 which correspond to those of other figures are given the same reference numerals.

The tube includes an electron gun 24, which is encapsulated in one end for the purpose of producing and directing a stream of electrons along the axis of the periodically loaded waveguide structure 10. A cooled collector is located in the enlarged enclosure 24 at the opposite end of the tube for the purpose of collecting electrons from the gun. Details of the collector and cooling mechanism are not shown as they may be conventional and do not form a part of the present invention.

The gun 26 consists of a cathode 27 and a cathode heater 28 which is connected to a suitable energizing source (not shown) and which causes the cathode to emit electrons which heated. A centrally apertured electron stream focusing electrode 7 and corresponding centrally apertured electron beam accelerating anode 8 are provided for causing the electrons emitted by the cathode 27 to be projected outwardly along the axis of the periodically loaded waveguide structure 10 in a stream as depicted by broken lines 29. In order to simplify the description and drawings, the energizing voltage supply for the electron gun electrode is not shown.

The elongated slow-wave structure is disposed between the electron gun and the beam collecting anode which is located at the opposite end of the tube. In order to insure stable operation of the tube (i.e., to prevent oscillations) an attenuator assembly 30 is disposed betweenthe ends of the slow-wave structure lit. The attenuator assembly illustrated includes two relatively thin annular discs 31 and 32 of lossy material having approximately the same cross section as the slow-wave circuit. The discs 31 and 32 are positioned on opposite sides of one of the loading tabs 12 to absorb incident and reflected electromagnetic Waves respectively and thereby effectively to electrically isolate the opposite-ends of the tube 25. The attenuator disc 32 and its position in waveguide 11 may best be seen by reference to FIGURE 6. The action of the attenuator assembly 34 and its structure are described in detail in United States Patent 2,939,993, filed in the name of Kurt E. Zublin and Robert A. Craig, Serial No. 632,841, issued June 7, 1960, entitled Traveling- Wave Tube Attenuators, and assigned to the assignee of the present invention. It is felt that a detailed description of these attenuators and their action is not warranted in the present patent application since they do not form a part of the present invention.

In operation, the electron stream 29 is directed through the drift tubes-16 along the axis of the periodically loaded slow-wave structure 10. Focusing of the stream throughout its travel along the structure is obtained by the periodic magnetic field as previously described. An electromagnetic wave is coupled onto the radio frequency circuit by means of the waveguide 33 at the left end of the structure. As the radio frequency field is propagated down the slow-wave structure 10 and the electron stream. is'directed down the structure, the radio frequency energy grows due to interaction between the electron stream and the radio frequency field in the well known manner.

In the interaction process, electrons in the stream 2% are redistributed with resultant bunching action. For all practical purposes, the electromagnetic wave incident on the attenuator assembly 30 is completely absorbed in the lossy disc terminating member 31. The prebunched or modulated electron stream 29 passes through the drift tube 16 of the attenuator assembly 30 and generates radio frequency waves in the portion of the radio frequency circuit beyond. Further interaction takes place between the stream 29 and the radio frequency energy in the output circuit. The amplified electromagnetic waves are taken out through the output waveguide 32.

Due to the fact that the loading tabs 12 bring the magnetic field so close to the axis of the structure 1d, the root mean square (R.M.S.) value of the axial magnetic field exhibits a pronounced increase with radius from the tube axis to the edge of the drift tubes lid. The Kit LS. axial magnetic field is about 1.5 as high at the Wall of the drift tubes 16 as on the axis of the structure. This variation in intensity of magnetic field over the radius of the structure lends a stiffness to the beam which is not achieved with either uniform field focusing or conven tional periodic focusing structures. This may be explained by the fact that as the electrons in the stream are redistributed by the radio frequency wave, it expands radially and as it expands it moves into an increased focusing field. Thus, less of the useful electrons are intercepted on the drift tubes 16 and the efficiency of the tube is increased. Further, heating of the drift tubes 16 and radio frequency circuit is reduced.

While particular embodiments of this invention have been shown and described, it will, of course, be understood that the invention is not limited thereto since many modifications both in the circuit arrangement and in the instrumentalities employed may be made. It is contemplated that the appended claims will cover any such modification as fall with the true spirit and scope of this invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A unitary structure for use as a magnetic and radio frequency electric circuit to produce radio frequency fields and a periodic magnetic field along the axis of the structure comprising a loaded Waveguide structure, said loaded waveguide structure including in combination a hollow cylindrical conductor member, a plurality of annular ring shaped magnets magnetized in a direction parallel to, said axis, and a series of magnetically permeable bars with centrally locatedapertures therethrough, said bars being longitudinally spaced apart along said conductor memher and extending therethrough, said bars being so positioned that the longitudinal axis of each succeeding bar intersects the axis of said conductor member at a ninety degree angle and is displaced by a predetermined angle relative to the axis of the preceding bar and the apertures through said bars are in alignment to define a tunnellike passage through the entire structure, each of said ring shaped magnets being coaxially positioned around said conductor member between each pair of adjacent bars of said series, each of the opposite faces of said magnets abutting a respective bar of the corresponding pair of bars, and a magnet means for applying a magnetic force of one magnetic polarity to the ends of alternate ones of said bars and a magnetic force of the opposite polarity to the remaining ones of said bars, wherein the magnetic polarity of said annular magnets at each face thereof is the same as the polarity of magnetic force applied to the one of said bars abutting such face by said magnet means. i

2. A magnetic circuit for focusing an electron stream which circuit includes at least one pair of magnet members, a series of annular ring shaped magnets and a series of longitudinally spaced bars of magnetically permeable material, said series of bars including two sets, each of said sets of bars having the longitudinal axes of its respective bars in a single plane which intersects a plane occupied by the longitudinal axes of the other set of bars along a line intermediate the ends of said bars, the bars of one of said sets being intercalated with bars from the other one of said sets, said magnet members being positioned on opposite sides of said series of bars and magnetically coupled thereto in such a manner that a magnetic force of one polarity is coupled to the opposite ends of one set of bars and a magnetic force of the opposite polarity is coupled to the opposite ends of said other set of bars, said ring shaped magnets being individually placed between each pair of adjacent ones of said bars and coaxially positioned with axes along the said line intermediate the ends of said bars, said ring shaped magnets being disposed With the opposite faces thereof abutting the corresponding bars, said ring shaped magnets being magnetized in a direction whereby each of said faces has the same magnetic polarity as the polarity of the magnetic force coupled to the one of said bars abutting thereto by said magnet members.

3. A unitary magnetic and radio frequency electric circuit for producing radio frequency fields and a periodic magnetic field along a common path including a periodically loaded waveguide structure having a series of longitudinally spaced bars of magnetically permeable material extending out through opposite sides thereof, said series of bars including two sets, each of said sets of bars having the longitudinal axes of its respective bars in a single plane which intersects a plane occupied by the longitudinal axes of the other set of bars along a line intermediate the ends of said bars, the bars of said sets being intercalated, at least one pair of magnet members positioned on opposite sides of said series of bars and magnetically coupled thereto in such a manner that a magnetic force of one polarity is coupled to the opposite ends of one of said sets of bars and a magnetic force of the opposite polarity is coupled to the opposite ends of said other set of bars whereby a periodic magnetic field is provided along the axis of said waveguide structure, and a series of annular ring shaped magnets magnetized in a direction parallel to said line and individually placed between each pair of adjacent ones of said bars and coaxially positioned with axes along the said line intermediate the ends of said bars, said ring shaped magnets being disposed with the opposite faces thereof abutting the corresponding bars, the magnetic polarity of said ring shaped magnets at each face thereof being the same as the polarity of the magnetic force coupled to the one of said bars abutting thereto by said magnet members.

4. A magnetic circuit for forming a periodic magnetic focusing field along an axis to be traversed by an electron stream comprising a series of bars of magnetically permeable material having apertures intermediate the ends of said bars, said bars being longitudinally positioned along the axis of the electron stream in such a manner that their longitudinal axes intersect the axis of the electron stream and the axes of the apertures are in alignment with the axis of the electron stream to permit the unimpeded passage of the stream, a magnetic means for applying a magnetic force of one magnetic polarity to the ends of alternate ones of said bars and 'a magnetic force of opposite polarity to the remaining ones of said bars, and an annular magnet disposed between each pair of adjacent bars of said series and positioned coaxially With respect to said axis, each of the opposite faces of said magnet abutting a respective bar of said pair, said magnet being magnetized in a direction parallel to said axis, the magnetic polarity of said annular magnet at each face thereof being the same as the polarity of magnetic force applied by said magnet means to the one of said bars abutting such face.

5. A magnetic circuit for forming a periodic magnetic focusing field along an axis to be traversed by an electron stream comprising a series of bars of magnetically permeable material having apertures intermediate the ends of said bars, said bars being longitudinally positioned along the axis of the electron stream in such a manner that their longitudinal axes intersect the axis of the electron stream and the axes of the apertures are in alignment with the axis of the electron stream to permit the unimpeded passage of the stream the longitudinal axis of each bar being disposed at right angles to the longitudinal axis of the adjacent bars, a magnetic means for applying a magnetic force of one magnetic polarity to the ends of alternate ones of said bars and a magnetic force of opposite polarity to the remaining ones of said bars, and an annular magnet disposed between each pair of adjacent bars of said series and positioned coaxial'ly with respect to said axis, each of the opposite faces of said magnet abutting a respective bar of said pair, said magnet being magnetized in a direction parallel to said axis, the magnetic polarity of said annular magnet at each face thereof being the same as the polarity of magnetic force applied by said magnet means to the one of said bars abutting such face.

References Cited by the Examiner UNITED STATES PATENTS 2,503,174 4/50 Reisner 31384 2,847,607 8/58 Pierce 31384 X 2,867,745 1/59 Pierce 313-84 X 2,876,373 3/59 Veith et a1. 3l3--84 2,956,193 10/60 De Wit 3l384 3,013,173 12/61 Sturrock 315-35 X FOREIGN PATENTS 596,064 7/59 Italy. 792,002 3/58 Great Britain.

GEORGE N. WESTBY, Primary Examiner.

RALPH G. NILSON, Examiner. 

3. A UNITARY MAGNETIC AND RADIO FREQUENCY ELECTRIC CIRCUIT FOR PRODUCING RADIO FREQUENCY FIELDS AND A PERIODIC MAGNETIC FIELD ALONG A COMMON PATH INCLUDING A PERIODICALLY LOADED WAVEGUIDE STRUCTURE HAVING A SERIES OF LONGITUDINALLY SPACED BARS OF MAGNETICALLY PERMEABLE MATERIAL EXTENDING OUT THROUGH OPPOSITE SIDES THEREOF, SAID SERIES OF BARS INCLUDING TWO SETS, EACH OF SAID SETS OF BARS HAVING THE LONGITUDINAL AXES OF ITS RESPECTIVE BARS IN A SINGLE PLANE WHICH INTERSECTS A PLANE OCCUPIED BY THE LONGITUDINAL AXES OF THE OTHER SET OF BARS ALONG A LINE INTERMEDIATE THE ENDS OF SAID BARS, THE BARS OF SAID SETS BEING INTERCALATED, AT LEAST ONE PAIR OF MAGNET MEMBERS POSITIONED ON OPPOSITE SIDES OF SAID SERIES OF BARS AND MAGNETICALLY COUPLED THERETO IN SUCH A MANNER THAT A MAGNETIC FORCE OF ONE POLARITY IS COUPLED TO THE OPPOSITE ENDS OF ONE OF SAID SETS OF BARS AND A MAGNETIC FORCE OF THE OPPOSITE POLARITY IS COUPLED TO THE OPPOSITE ENDS OF SAID OTHER SET OF BARS WHEREBY A PERIODIC MAGNETIC FIELD IS PROVIDED ALONG THE AXIS OF SAID WAVEGUIDE STRUCTURE, AND A SERIES OF ANNULAR RING SHAPED MAGNETS MAGNETIZED IN A DIRECTION PARALLEL TO SAID LINE AND INDIVIDUALLY PLACED BETWEEN EACH PAIR OF ADJACENT ONES OF SAID BARS AND COAXIALLY POSITIONED WITH AXES ALONG THE SAID LINE INTERMEDIATE THE ENDS OF SAID BARS, SAID RING SHAPED MAGNETS BEING DISPOSED WITH THE OPPOSITE FACES THEREOF ABUTTING THE CORRESPONDING BARS, THE MAGNETIC POLARITY OF SAID RING SHAPED MAGNETS AT EACH FACE THEREOF BEING THE SAME AS THE POLARITY OF THE MAGNETIC FORCE COUPLED TO THE ONE OF SAID BARS ABUTTING THERETO BY SAID MAGNET MEMBERS. 